Asumi Aida is a cheerful high school student who lives a parallel life inside an online fantasy role-playing game called The World R:X. There her username is Sakuya. While playing together with her classmates Ikuta Iori (username Tobias) and Etou Eri (username Mary) inside the game and having an ordinary treasure hunting adventure, they end up caught into a ominous situation. After an unfortunate encounter with an elite guild and a mysterious player named Hermit, Etou loses consciousness and for an unknown reason is unable to wake up.
Tobias, Mary, and Sakuya challenge the impregnable "The One Sin", but they lose their way in the maze and unintentionally trap other guild members. A mysterious cat is watching their blunder.
Tobias, Mary, and Sakuya on their way to find some treasures inside a dungeon, they see a guild trying to fight against "The One Sin", but while going around to see the fight, they unintentionally activate a trap that kills half the guild members and makes "The One Sin" to attack them. Meanwhile a mysterious cat with a stick is watching their blunder.